The first chestnut here is the appearance of a Trojan Number, so called because it is the stratagem by which authors infiltrate their findings into the columns of the media. In this case it is an impressive 400,000, which is the number of babies said to be involved in the study. In fact, almost all of them have no part in the study at all, as they are normal, healthy births.

As I wrote in a book called Sorry, Wrong Number! in 2000, chlorine is essential to life on earth, not only in the form of its sodium salt, but as a constituent of more than more than 1500 vital compounds in plants and animals, including our digestive juices. The chlorination of drinking water has saved more human lives than any other hygienic measure.

However in 1991, Greenpeace activist Christine Houghton said: "Since its creation, chlorine has been a chemical catastrophe. It is either chlorine or us." Even by Greenpeace standards this was a pretty remarkable piece of ignorant, hysterical nonsense. When chlorination was stopped in Peru in 1991 as a result of pressure from the EPA and Greenpeace, an epidemic broke out that spread through Latin America. Some 800,000 people became ill with cholera and 6,000 people died. Millions of people are still dying all over the world because of dirty water.

The anti-chlorine movement was one of the many legacies of Rachel Carson. It was intensified by an EPA study in the mid 1980s that purported to show that one of the by-products of chlorination (trihalomethanes) was carcinogenic. This involved subjecting hapless rodents to very high concentrations.

That was a classical piece of junk epidemiology, based on accidental correlation, of the sort that editors cannot resist. Take just one of the conditions mentioned.

Anencephalus is so rare that most people have never heard of it. Its frequency is less than two per ten thousand of live births, so the impressive number whittles down to something under 80 actual cases. These are then divided into at least two groups - those who are exposed to the putative cause (at an arbitrary threshold) and those who are not. So the whole claim is based on a group of less than 40 babies - unlikely to produce a significant result, even with the debased statistical standards used by modern epidemiology.

Then there is the measure of exposure itself. How much of the dreaded fluid did the pregnant women drink? How did the boffins distinguish between a thirsty mother in a low dosage area and a non-thirsty mother in a high dosage area?

The other glaring defect is that this is clearly a Data Dredge, given away by the fact that three conditions are mentioned. How many others were looked at we are not told.

The abysmal standard of significance in modern epidemiology is a one in 20 chance of the result having occurred by accident. But if you look at ten different diseases, this standard means that the probability of at least one crossing a given threshold of risk level becomes 40 per cent, which should be adjusted for, but isn’t. As for the threshold itself, for a variety of reasons such as confounding factors, most scientists would be looking for more than a doubling of risk before claiming significance.

Who now believes that drinking tap water causes cancer? Yet 6,000 Peruvians died because of that claim, which was subsequently withdrawn. Fortunately, such scares (and miracle breakthroughs) are now so frequent that ordinary people have become blasé about them - they yawn and turn to the sports pages. But there is no accounting for what politicians will do.

Like footballers, epidemiologists talk in clichés. After a while you can predict what they are going to say:

"The biological mechanism for how these disinfection by-products may cause defects are still unknown"

And:

"...more research needs to be carried out to determine these side-effects."

The establishment media go through the ritual of publishing this nonsense. Hardly a day goes by without at least one scare or breakthrough. They are just page fillers, but there is always the danger that someone will take them seriously. As for the epidemiologists, irresponsible is an inadequate word.

Reel off a few acronyms (DDT, HRT, MMR for example) and you uncover stories of millions of unnecessary deaths and lives turned to misery, all caused by the rejection of the boons of scientific research because of mindless attacks. ®

Professor John Brignell has edited the popular NumberWatch site since 2000 and is the author of Sorry! Wrong Number and The Epidemiologists: Have they got scares for you!. He was for ten years Reader in Electronics at The City University and held the Chair in Industrial Instrumentation at Southampton for 20 years from 1980.